Linking water quality with plankton dynamics: multivariate analyses of environmental drivers in Lake Koka

Lake Koka, one of Ethiopia's major freshwater reservoirs, is facing rapid ecological degradation due to anthropogenic pressures such as agricultural runoff, industrial effluents and domestic waste discharges. This study examined the relationship between water quality parameters and plankton dynamics across different ecological zones of the lake, such as the Pelagic, littoral, vegetated, and river inlet using principal component analysis (PCA). Field sampling was conducted during the dry season months of January and March 2025, and the water samples were analyzed for key physicochemical parameters, including dissolved oxygen, pH, temperature, electrical conductivity, turbidity and nutrients such as nitrate, ammonia, soluble reactive phosphorus and total phosphorus. The phytoplankton and zooplankton communities were identified and quantified using standard microscopic techniques. The PCA results revealed that the phytoplankton assemblages were strongly correlated with total phosphorus, soluble reactive phosphorus and pH, particularly in the vegetated zone, while the river mouth zone was associated with high ammonia and conductivity, indicating external nutrient loading. Zooplankton groups exhibited distinct spatial patterns, with Rotifera dominant in nutrient-rich inflow zones and Copepoda and Cladocera being abundant in pelagic and littoral regions with high nitrate availability. These spatial associations underscore the strong influence of nutrient enrichment and water chemistry on plankton distribution and lake trophic dynamics. The findings highlight that nutrient-driven eutrophication is the main driver of community structure in Lake Koka, emphasizing the need for integrated catchment and water quality management strategies to restore ecological balance and sustain biodiversity.